1. Field of the Invention
The present invention relates generally to combustible sensors and, more particularly, to a selective combustible sensor and method for measuring the concentration of combustibles in a gaseous atmosphere.
2. Description of the Related Art
Solid-state electrochemical cells based on oxygen-ion conducting zirconium dioxide (ZrO.sub.2) have found extensive use as oxygen sensors in many applications. One of these is in the automotive vehicle industry, where the oxygen sensor is used to control the air-to-fuel (A/F) ratio of an engine by measuring oxygen partial pressure in engine exhaust. Typical automotive oxygen sensors consist of a single ZrO.sub.2 cell operated in an open-circuit mode (Nernst cell) and are used for stoichiometric A/F ratio control. Oxygen sensors more sensitive than the Nernst cell are based on the oxygen-pumping principle, i.e., the application of an external current to transfer (pump) oxygen from a negative potential side to a positive potential side of the ZrO.sub.2 cell. Automotive oxygen sensors of this type are designed for operation with lean A/F ratio mixtures. These sensors are commonly called lean exhaust gas oxygen, LEGO, sensors.
Examples of such LEGO sensors are disclosed in U.S. Pat. Nos. 4,272,329, 4,272,330 and 4,272,331 to Hetrick et al. and assigned to the same assignee as the present invention. These patented LEGO sensors, which are immersed in a gaseous atmosphere, are constructed to define an enclosed volume which communicates with the atmosphere by way of a small aperture. The above-referenced patented LEGO sensors include two electrochemical cells which define the enclosed volume. One cell is called the pump cell while the other cell is called the sensor cell. When attached to an external power supply, a current (Ip) through the pump cell either adds or removes (from or to the atmosphere) gaseous oxygen from the enclosed volume. As a result of the pumping action, an electromotive force or e.m.f. (V.sub.s) develops across the sensor cell which can be used to measure the change in oxygen partial pressure in the enclosed volume relative to the atmosphere.
Although the above LEGO sensors have worked well, they have been used at constant temperature and V.sub.s. As a result, these sensors have not been used to selectively detect combustible gases in a gaseous atmosphere. Also, these sensors have not been used to detect one combustible gas in the presence of other combustible gases in a gaseous atmosphere.